This invention relates to a method for detecting and processing ultrasonic Doppler signals in order to display a blood flow image and a tomogram by detecting echo signals using a plurality of receiving delay circuits during a period that the ultrasonic beam is repeatedly transmitted in the same direction, and to a system for executing that method.
The recent ultrasonic diagnosis apparatus with ultrasonic probe containing a number of transducers, are capable of displaying a tomogram and a blood flow image as obtained from the two dimensionally detected Doppler signal, in real time, at the same time.
To obtain the Doppler signal, it is necessary to analyze the received echo signal. To improve the frequency resolution of the detected doppler signal, transmitting and receiving of the ultrasonic beam must be repeated in the same direction. The frequency resolution "df" is given by EQU df=1/(Tr.(n-1)) (1)
where Tr is the repeating period of the transmitting pulse signal, and "n" is the repeating number. As seen from this equation, as the repeating number "n" is increased, the frequency resolution "df" becomes small, that is, the frequency resolution is improved. When the repeating number "n" is increased, the two dimensional Doppler signal is detected, and this is displayed in real time, the number of frames of the blood flow image and the ultrasonic tomogram are reduced.
To solve this, there is a proposal to arrange the receiving system in parallel. In this proposal, a transmitting beam is received with different delay times, so that two raster signals, for example, are obtained by the transmitting beam in the same direction. This parallel arranged receiving system needs two circuits each comprising a detector for obtaining ultrasonic tomogram, a filter, A/D converter, and the like. Practically, however, it is difficult to match the characteristics of these circuits. Generally, each of these circuits uses a logarithmic circuit of approximately 100 dB at the prestage of the detector, in order to make a gray scale display. To obtain a sensitivity difference within 1 dB in the linear circuit, it must be set within 0.1 dB in the logarithmic circuit. Presence of the sensitivity difference between these two circuit systems would cause the sensitivity of the raster signal to be not uniform, and to decrease an image quality.
For the above reasons, it has been demanded that with the parallel arrangement of the receiving circuit, the blood flow image and the tomogram are displayed by obtaining the Doppler signal and the raster signal at the same time, and a single circuit for processing the raster signal is used for the image quality improvement.